Non-aqueous electrolyte secondary batteries, in particular, lithium ion secondary batteries have been widely used in mobile phones and notebook personal computers. The non-aqueous electrolyte secondary batteries are now indispensable for the present ubiquitous network society, for which further improvement in capacity is eagerly desired. Furthermore, the non-aqueous electrolyte secondary batteries have been employed as power sources for power tools, and are now greatly expected as future power sources for hybrid automobiles.
During these 10 years since the mass production of lithium ion secondary batteries started in 1991, improvement in battery structure has advanced and the technique of high-density filling has reached close to the limit of improvement. As a result, the battery energy density has been doubled from 280 Wh/L to 580 Wh/L. However, there has been no change in the basic configuration that uses LiCoO2 for a positive electrode active material and graphite for a negative electrode active material. As such, the expectation is placed on the development of new materials satisfying the requirements for high capacity, high performance, and high safety.
Under these circumstances, studies have been actively conducted on lithium-containing transition metal oxides containing nickel and manganese represented by LixNi0.5Mn0.5O2 (1≦x) (hereinafter referred to as nickel-manganese based oxides) as promising materials to replace LiCoO2. However, because of the difficulty in synthesis thereof, it appears that various types of nickel-manganese based oxides having different electrochemical characteristics have been synthesized by various methods among researchers. As such, the precise structure of LixNi0.5Mn0.5O2 has not yet been determined.
On the other hand, with respect to LiCo1/3Ni1/3Mn1/3O2 containing Co, the synthesis thereof is comparatively easy and the structure thereof has almost been finally determined. Lithium ion secondary batteries using this material have already been manufactured on a mass production basis as a cylindrical battery for notebook personal computers (size 18650), a prismatic battery for mobile phones, and a cylindrical battery for power tools (size 26650).
LixNi0.5Mn0.5O2 has a high capacity equivalent to or better than that of LiCo1/3Ni1/3Mn1/3O2, the safety of which is expected to be dramatically improved. Moreover, LixNi0.5Mn0.5O2 can be inexpensively produced because it does not contain expensive Co. However, a LixNi0.5Mn0.5O2 that is more excellent than LiCo1/3Ni1/3Mn1/3O2 has not yet been achieved because of the disadvantage of the difficulty in synthesis. In view of the above, in order to achieve both a high level of safety and a high capacity, the following proposals have been suggested.
Nickel and cobalt have analogous chemical characteristics. Accordingly, LiCoO2 and LiNiO2 are capable of readily forming a solid solution represented by LiNi1−xCoxO2 (0<x<1) in the whole domain of x. However, nickel and manganese have chemical characteristics different from each other. For this reason, it has been difficult to form a lithium-containing transition metal oxide in which nickel and manganese are dispersed in the atomic level by the conventional method, namely, by baking to allow solid phase dispersion to occur.
In order to solve this problem, one proposal suggests a method for preparing an active material including the steps of synthesizing Ni0.5Mn0.5(OH)2 by coprecipitating the same number of moles of nickel and manganese, mixing the resultant hydroxide with LiOH.H2O, and baking the mixture in air at a high temperature of 1000° C. (see Non-patent Document 1). By doing this, the Ni ions and the Mn ions, which are normally present as ions with a valence of 3+ in LiNi0.5Mn0.5O2, are allowed to be present as Ni2+ and Mn4+, making it possible for the both ions to be dispersed without being isolated from each other. As a result, an active material having a high capacity of approximately 150 mAh/g can be obtained. This active material provides a charge-discharge curve showing a high electric potential approximate to that of LiCoO2. In other words, an active material different from the conventional nickel-based material represented by LiNi1−xCoxO2 (0<x<1) that provides an S-shaped charge-discharge curve showing a low electric potential can be obtained.
Non-patent Document 2 gives a detailed examination with regard to the mechanism of electrochemical reaction of LixNi0.5Mn0.5O2 (x=1) using methods of Neutron Diffraction, Li6 MAS NMR (Magic-Angle Spinning Nuclear Magnetic Resonance) spectroscopy, the first-principle simulation, and the like. This document reports that Ni ions migrate from one site to another in the solid matrix in association with charge and discharge (oxidation and reduction). This documents further reports that in association with the migration of Ni ions, Li sites are decreased during discharge, resulting in a large irreversible capacity in the initial charge.
Non-patent Document 3 suggests a material represented by Li[NixLi1/3-2x/3Mn2/3-x/3]O2 (0<x≦½). This is a material in which transition metal sites are partially substituted with Li. It is attempted to achieve an improvement in capacity by extracting lithium and oxygen from the skeleton of the material containing lithium in excess in the first charge. In the case of the active materials of Non-patent Documents 1 and 2, the ratio between the total number of moles of transition metal elements and the number of moles of lithium element is 1:1. In the case of the active material of Non-patent Document 3, the number of moles of lithium element is greater than the total number of moles of transition metal elements, that is, lithium is contained in excess.
Non-patent Documents 4 and 5 suggest a material represented by xLi2MnO3.(1-x)LiMO2 (M=Ni, Mn). This material, as in the case of Non-patent Document 3, contains lithium in excess. In the crystal of this material, Li2MnO3 domain and LiMO2 (M=Ni, Mn) domain are present in a complex state. In Non-patent Documents 4 and 5 also, as in Non-patent Document 3, lithium and oxygen are extracted from the skeleton of the material in the first charge.
The disclosure in Patent Document 1 is similar to those in Non-patent Documents 3 to 5. FIG. 9 in Patent Document 1 shows charge-discharge curves, in which the initial charge-discharge curve is significantly different from the second and subsequent charge-discharge curves. In the case of Non-patent Documents 3 to 5 and Patent Document 1, a flat portion appears around from 4.5 V to 4.6 V in the first charge. It is considered that extraction of lithium and oxygen occurs in this flat portion. In such a material containing lithium in excess, since lithium and oxygen are extracted from the skeleton in the first charge, a large irreversible capacity is generated.
Non-patent Document 4 suggests an acid treatment for suppressing the increase in the irreversible capacity. That is, extracting lithium and oxygen beforehand from the skeleton of the material by the acid treatment can prevent the generation of irreversible capacity in the first charge after the fabrication of a battery.    Non-patent Document 1: T. Ohzuku, and Y. Makimura, Chem. Lett., 744 (2001)    Non-patent Document 2: J. Breger, Y. S. Meng, Y. Hinuma, S. Kang, Y. Shao-Horn, G. Ceder, and C. P. Grey, Chem. Mater., 18. 4768(2006)    Non-patent Document 3: Zhonghua Lu, L. Y. Beaulieu, R. A. Donaberger, C. L. Thomas, and J. R. Dahn, J. Electrochrem. Soc., 149(6). 778(2002)    Non-patent Document 4: M. M. Thackeray, C. S. Jonson, J. T. Vaughey, N. Li, and S. A. Hackney., J. Mater. Chem., 15. 2257(2005)    Non-patent Document 5: M. M. Thackeray, S.-H. Kang, C. S. Jonson, J. T. Vaughey, N. Li, and S. A. Hackney., Electrocem. Commun. 8. 1531(2006)    Patent Document 1: WO 2002/078105